The present invention relates to an ink for producing catalyst layers for electrochemical devices. A catalyst ink which contains at least one organic solvent having functional groups or substituents which are stable to oxidative degradation is described. The ink is used in the production of catalyst layers and electrodes for electrochemical devices, in particular for fuel cells, membrane fuel cells (PEMFCs, DMFCs), electrolyzers or sensors.
Fuel cells convert a fuel and an oxidant at separate locations at two electrodes into electric power, heat and water. As fuel, it is possible to use hydrogen, a hydrogen-rich gas or methanol, while oxygen or air serves as oxidant. The process of energy conversion in the fuel cell has a particularly high efficiency. For this reason, fuel cells (PEMFCs, SOFCs, etc) are becomingly increasingly important for mobile, stationary and portable applications. Membrane fuel cells (PEMFCs, DMFCs, etc.) are particularly suitable for use in the abovementioned fields because of their compact construction, their power density and their high efficiency.
The key component of a PEM fuel cell is the membrane-electrode assembly (MEA). The membrane-electrode assembly has a sandwich-like structure and generally comprises five layers.
A schematic structure of a five-layer membrane-electrode assembly is shown in FIG. 1. Here, the anode gas diffusion layer (1) together with the anode catalyst layer (2) forms the gas diffusion electrode (GDE) on the anode side; the cathode gas diffusion layer (5) together with the cathode catalyst layer (4) forms the gas diffusion electrode (GDE) on the cathode side. The ionomer membrane (3) is located between the two GDEs.
The structure of a three-layer catalyst-coated membrane (CCM) is shown in FIG. 2. Here, the catalyst layers (2) and (4) are applied directly to the membrane (3).
In the production of a five-layer MEA, it is usual to apply two catalyst-coated gas diffusion layers (or gas diffusion electrodes, GDEs) to the front side and the rear side of an ionomer membrane (3) and press them together to produce an MEA. However, processes for producing MEAs using catalyst-coated ionomer membranes (CCMs) are also known. In this case, the CCMs are generally combined with the gas diffusion layers (1) and (5) which have not been coated with catalyst.
The present invention describes novel catalyst-containing inks and pastes which can be used for coating various substrates (e.g. ionomer membranes, gas diffusion layers, carbon fibre nonwovens, polymer films, release films, etc.). Such coated substrates are, for example, used for producing electrodes or membrane-electrode assemblies for fuel cells.
Various compositions for catalyst inks are known from the patent literature.
Thus, EP 797 265 B1 describes an ink for producing membrane-electrode assemblies for fuel cells. Solvents used are isopropanol and glycerol.
U.S. Pat. No. 5,869,416 describes alkylene carbonates, for example propylene carbonate, as solvents for catalyst inks.
EP 622 861 B1 discloses electrode inks containing alkoxy propanols or aryloxypropanols as organic solvents.
EP 945 910 A2 describes inks comprising two immiscible organic solvents A and B. As solvents A, use is made of monohydric or polyhydric alcohols, glycols, glycol ether alcohols, glycol ethers and mixtures thereof. The solvents B are nonpolar hydrocarbons or slightly polar solvents. Tertiary alcohols are not described.
EP 309 337 A1 discloses an electrode ink containing alcohols and water. The ionomer is dissolved in a mixture of water and ethanol, or isopropanol.
EP 785 588 A2 describes electrode inks containing sublimable pore formers. Cyclohexanol is used as alcoholic solvent.
EP 731 520 A1 teaches an ink comprising a catalyst, ionomer and solvent, with water being used as solvent. This ink contains no further organic components apart from the ionomer.
Furthermore, EP 945 910 A2 discloses catalyst inks containing linear dialcohols (e.g. ethylene glycol, propylene glycol) and water.
WO 2004/054021 describes inks which comprise polar aprotic organic solvents.
Inks which contain primary and/or secondary alcohols (e.g. alkanediols, glycols or glyercol) as solvents have fundamental disadvantages; in particular, they have an unsatisfactory stability. The primary or secondary OH groups gradually react in the ink in the presence of the catalyst and are subject to oxidative attack or degradation in the presence of atmospheric oxygen (for which even traces suffice). They can in this way be oxidized further to, for example, aldehydes, ketones and carboxylic acids. This oxidation process can lead to various degradation products in the ink.
The oxidative degradation of alkanediols is known from the literature. The secondary or primary alcohol groups are in this case oxidized first. In further stages, these intermediates can then be oxidized to, for example, oxalic acid (C2H2O4), lactic acid (C3H6O3), pyruvic aldehyde (C3H4O2) or pyruvic acid (C3H4O3), and autocondensation can finally occur to form carbon dioxide, carbon monoxide and acetic acid.
The organic acids formed in this decomposition process can, like their salts, remain in the catalyst layer or on the surface of the catalyst. They can lead to a deterioration in performance of the catalyst layer or the membrane-electrode assembly and may have to be removed from the catalyst layers by labourious cleaning processes.
Furthermore, the oxidation process described can influence the storage stability of the catalyst inks and cause a change in the viscosity of the ink during storage.
It was therefore an object of the present invention to provide a catalyst ink which has a high stability in respect of decomposition reactions and a good storage stability. The formation of degradation or decomposition products in the ink should be prevented. The catalyst layers produced therewith should have a high performance. The ink should considerably simplify the production process for electrodes and make post-treatment and cleaning steps for the electrode layers superfluous. Depending on the coating methods selected, the ink solvents should be miscible with water and/or the ionomer solution, have an appropriate boiling point and a suitable evaporation number (EN).